This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-076354, filed Mar. 19, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a plasma processing method and a plasma processing,apparatus for processing a subject by plasma generated by high-frequency discharge.
Conventionally there is a reactive ion etching (RIE) method as one of dry etching methods widely used for micromachining in a process of manufacturing semiconductor elements. As the RIE method, magnetron RIE is known in which a magnetic field is employed as plasma to increase the density of the plasma and thus achieve high-speed etching and high-precision micro-machining (U.S. Pat. No. 5,444,207; Jpn. Pat. Appln. KOKAI Publication No. 6-53117; Preliminary Report on 13th Dry Process Symposium of the Institute of Electrical Engineers of Japan, Tokyo, 1991, pp 99-103).
A prior art magnetron plasma processing apparatus will now be described taking a magnetron etching apparatus shown in FIG. 1 as an example.
Referring to FIG. 1, the apparatus includes an anode mounted on an upper electrode 2 of an upper inner wall of an evacuated vessel 1 and a lower electrode (cathode) 3 opposed to the upper electrode 2 and serving as a support table for supporting a substrate to be processed. Power is generated from a high-frequency power supply 31 and applied between the upper and lower electrodes 2 and 3 through a matching circuit 32.
An electric field E is generated by the electrodes 2 and 3 to form plasma therebetween, and a self-bias electric field is induced on the surface of the lower electrode 3 and thus accelerated reactive ions collide with the surface of the substrate from the plasma, with the result that an etching reaction proceeds.
In the magnetron RIE, a magnetic field B is generated from a dipole ring 9 in a direction perpendicular to the self-bias electric field. In FIG. 1, the magnetic field B is shown in schematic form. If the magnetic fields E and B intersect each other, electrons in the plasma can be drifted in the Exc3x97B direction by the Lorentz force. By causing the electrons to travel a long distance by the drift, they come into collision with neutral molecules and atoms with higher frequency, and plasma density is increased. If electrons are confined in plasma by applying a magnetic field and lengthening their lifetime (a time period required until the electrons come into collision with the inner wall of an evacuated vessel, the electrodes and the substrate to be processed), plasma density can be increased further.
The above high-density plasma not only improves an etching rate but also inhibits a radical and a film to be etched from reacting with each other (isotropic reaction). Consequently, even though the pressure of gas lowers, damage can sufficiently be lessened and ion energy, which decreases a selective ratio (of the film to be etched to an underlying film or a mask), can sufficiently be lowered.
The magnetron RIE is currently used for various types of thin film processing since it has excellent characteristics as described above. However, the magnetron RIE has the following problem in a micro-loading effect. Since, in the magnetron RIE, a magnetic field is applied, the directivity of ions incident on a substrate to be processed is disordered and thus the ions enter the substrate obliquely, with the result that an etching rate is lowered in high-anisotropy etching and in etching for patterns whose processing size is small or whose aspect ratio is high.
In the prior art magnetron RIE apparatus as shown in FIG. 1, it is a single high-frequency power supply that is connected to the cathode and the single high-frequency power supply is used to both generate plasma and control the energy of ions incident upon the substrate. Therefore, the energy of ions incident upon the substrate cannot be controlled independently of the density (Ne) of plasma generated between the electrodes and a control range of process is narrowed accordingly.
An object of the present invention is to provide a plasma processing method capable of controlling plasma density and ion energy independently of each other.
Another object of the present invention is to provide a plasma processing apparatus capable of controlling plasma density and ion energy independently of each other.
According to a first aspect of the present invention, there is provided a plasma processing method for applying high-frequency power between upper and lower electrodes opposed to each other in a processing chamber to generate plasma and processing a subject placed on the lower electrode by the plasma,
wherein the power applied to the lower electrode has at least two frequencies including a first frequency which is proportional to plasma density and a second frequency a square of which is proportional to the plasma density, thereby generating an electric field intersecting a surface of the subject substantially at right angles.
According to a second aspect of the present invention, there is provided a plasma processing apparatus comprising:
upper and lower electrodes opposed to each other in a processing chamber;
a first power supply for generating power having a frequency which is proportional to a frequency characteristic of plasma density and applying the power to the lower electrode;
a second power supply for generating power having a frequency a square of which is proportional to the frequency characteristic of plasma density and applying the power to the lower electrode; and
magnetic-field generation mechanism generating a magnetic field between the upper and lower electrodes.
In the present invention described above, power having a frequency which is proportional to the plasma density and a frequency the square of which is proportional to the plasma density is applied to a single lower electrode. Thus both a frequency for controlling the plasma density and a frequency which ions of plasma can follow independently of the plasma density are provided and consequently high-frequency power for controlling plasma generation and high-frequency power for controlling energy of ions incident upon a subject to be processed can be separated from each other, with the result that the ion incident energy can be controlled independently while making the plasma density uniform. The process control range is therefore expanded to suppress the micro-loading effect in which the etching rate lowers as the etching size reduces.
As compared with a conventional single-frequency power application, the angles of ions incident upon a subject to be processed are inhibited from being varied, and the variations in etching rate due to a pattern size are eliminated by performing etching as plasma processing. The etching thus proceeds in a region where an etch-stop occurred and the etch-stop is cancelled accordingly.
Since it is a single lower electrode that high-frequency power is applied to, high-density plasma can be generated near a subject placed on the lower electrode, with the result that plasma processing is improved in efficiency.
Since, moreover, two frequencies of 27.12 MHz or higher and 5.424 MHz or lower are superposed on each other, high-frequency power having the superposed frequencies can be applied to the same electrode in consideration of control conditions of high- and low-pass filters. The energy of ions can be controlled by the power of 5.424 MHz or lower.
The electric field E and magnetic field B intersect at right angles, so that electrons in plasma can be drifted in the Exc3x97B direction by the Lorentz force and caused to travel a long distance. The plasma density is therefore improved.
The plasma processing is performed by DRM (depositional remanent magnetization) or using a magnetic-field generation mechanism capable of rotating around a plasma generating region. Thus, a magnet for applying a magnetic field can be decreased in size and accordingly the apparatus can be miniaturized as a whole.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.